Plasma arc torches are widely used in the cutting or marking of metal workpieces. A plasma arc torch generally includes a torch body, an electrode mounted within the body, a nozzle with a central exit orifice, electrical connections, passages for cooling and arc control fluids, a swirl ring to control the fluid flow patterns, a power supply and a means to control the operation of the torch.
A plasma arc torch generates a constricted ionized jet of plasma gas with high temperature and high momentum. The plasma jet typically exits the nozzle orifice of the torch typically at a temperature of between 15,000-50,000 degrees centigrade at a typical velocity of up to 7,000 m/s. A plasma jet with these temperatures and velocities produces extreme rates of heat transfer which are necessary to cut rapidly through metals. Such extreme rates of heat transfer can result in a serious hazard of traumatic burns to the operator and other workers in close proximity to the jet exiting the plasma arc torch. The operating instructions for commercial plasma arc torches usually instruct the operator to disable the power supply before disassembling the torch for service or when leaving the plasma arc torch unattended and warn the operator of possible injuries (e.g., electric shock) resulting from a failure to follow correct operating procedures. It is, however, frequently observed in practice that operators fail to follow the manufacturer's instructions and warnings.
Some plasma arc torches, such as handheld torches, can be trigger-activated devices. That is, the torch generates a plasma stream in response to operator activation of a trigger. Accidents can occur when the operator handles the torch in certain scenarios without turning off the power supply (e.g., changing consumables) or inadvertently activates the trigger controlling the plasma stream (e.g., to inadvertently fire the torch). Accidents can also occur with plasma arc torches when the operator places the torch body on a hard surface while attending to other tasks. In this event, the trigger can be inadvertently activated if objects like clothing or building materials are placed on top the torch. For example, in some cases, this can occur when a torch is placed in a user's pocket, such as while climbing or negotiating uneven terrain. Additionally, accidents can occur if a torch is dropped such that its trigger strikes a solid object.
Further, operators sometimes fail to disable the plasma arc torch power supply when changing consumable parts from the torch body. Replacement of consumable components often requires gripping the torch body, in the proximity of the control switch, while removing the torch cap to access consumable parts. Most plasma arc torches contain safety devices (e.g., cap sense switches that can detect presence or absence of certain consumables) so that, if the torch is disassembled, the power supply and thus the torch is deactivated. Unfortunately, these safety devices occasionally fail and sometimes indicate to the power supply that the cap is connected when it is not. In this event, if the operator removes the cap to exchange consumable parts and inadvertently compresses the trigger, the operator can be severely burned if and when the cap sense switch fails to prevent the torch from firing.